A gear of this type is known from German Patent No. 3 419 477, and reference will be expressly made herein to the embodiments specified in that document, without, however, limiting the present scope to them.
The special characteristic of the gear is that the strings, when linked together inside the guide, can transmit considerable tensile and compressive forces without generating much friction against the walls of the guide. Inside the guide the two strings are combined into a single intermediary component, to which the component that is to be shifted, the panel of a door for instance and especially the panel of a garage overhead door or similar structure, is connected.
The main concept behind these gears, which usually have continuous motion-transmission components, chains for example, although they may also involve discontinuous components as in the aforesaid patent, is that the engagement between the rotary drive mechanism and the motion-transmission component is disestablished at no point along the path of motion. Limiting switches are appropriately positioned to determine the back-and-forth motion along its intended path. The edges of the door panel are provided with a safety measure in the form of sensor strips that, when anything gets caught in the panel's path, terminate or reverse its motion. It is desirable to be able to deliver a gear of this type as completely assembled as possible to where it will be used to shorten and simplify the job of installing it on site as much as possible, eliminating in particular the necessity of intervention into the mechanism itself. In particular, if the rotary drive mechanism is engaged with the motion-transmission component when the gear is completely assembled, careless activation of the motor, in the wrong direction for example etc., can lead not only to injury but also to destruction of the parts involved.
Furthermore, many emergency means of preventing the mechanism from exceeding one of its limiting positions have been considered in order to avoid damage to the gear. Thus, an extra limit switch has been provided farther along the path of motion, demanding more expense and an according extension of the path. It is also possible to provide an overload switch that executes a mechanical motion against the force of a spring, whereby the resistance to displacement offered by the spring must be higher than the stress on the gear during normal operation. The electric motor can also be protected from overload by monitoring the electric parameters. In the last two cases, however, it is impossible to prevent the mechanical load exerted on the gear in such a situation from being much higher than in normal operation. This means that the components subjected to stress must be very hefty to eliminate the risk of their being destroyed.
The space required for feeding and introducing the separated strings is an especially important factor in allowing the gear to be delivered as completely assembled as possible even when the demand for space during shipment means that it must be dismantled at least into a drive component and a guide component. The aforementioned patent accordingly discloses not only feeding the strings parallel to the guide but also accommodating each separate string in a drum-type storage unit on each side of the point at which they unite.
In addition to the desirability of minimalizing the friction of the motion-transmission component inside the guide to the greatest extent possible, how easily and with how little friction each separated string can be moved in or out of its stored position is also significant, to keep the power consumption and requisite output of the motor low.